One in three US young adults still use tobacco, and college students are early adopters of emerging tobacco products (e.g., e-cigarettes, other vaping devices, hookah). Daily marijuana use in this population is the highest rate it has been in 35 years, and 40% of young-adult tobacco users report using marijuana in the past month. The majority of young people?s exposure to tobacco marketing occurs in the retail environment, and omnipresent advertisements and displays are a risk factor for smoking. Our preliminary research documented the co-marketing of cigars and marijuana: 327 of 531 stores (62%) that sold cigars had products that were labeled as blunts, blunt wraps, or marketed marijuana as a ?concept flavor? (e.g., Purple Haze, Cali Green, OGK cigarillos). Such marketing may encourage dual use and concurrent use of tobacco and marijuana, but this has not been studied. The proposed research addresses significant gaps in knowledge about the retail availability and co-marketing of tobacco and marijuana in a context of greater tobacco regulation, product diversification, and marijuana deregulation. California was the second state to pass ?Tobacco 21? and is among the 8 states that legalized recreational marijuana use, with retail sales beginning in 2018. California?s community colleges and public universities enroll 4 in 10 young adults (ages 18-24) in the state and serve a racially/ethnically diverse population. Currently, 34% of the 113 community colleges and 49% of the 33 public universities have tobacco-free or smoke-free policies that include emerging tobacco products. Integrating data from people, places and policies, four coordinated study aims will: (Aim 1) Test for spatial clustering of vape shops, hookah bars, other licensed tobacco retailers and marijuana retailers near public colleges/universities (n=146) and examine whether local smoke-free policies moderate change over time in retailer density near campuses; (Aims 2&3) Conduct annual assessments of retail marketing for emerging tobacco products and marijuana co-marketing in a statewide random sample of vape shops (n=600) and a random sample of licensed tobacco retailers (n=900) within one mile of the college campuses, then model changes over time as a function of campus policy and local tobacco control policy (e.g., smoke-free/vape-free air, retail licensing and sales restrictions); (Aim 4) Concurrent with marketing surveillance, assess tobacco and marijuana use among students (ages 18-24) at the campuses in an on-line survey conducted the first year that retail marijuana sales begin and two years later. Multilevel modeling will test whether greater retail availability and prevalence of marijuana co-marketing predicts higher odds of student use, and whether campus policies moderate these associations, controlling for campus enrollment characteristics and local policies. This research represents the first comprehensive assessment of marijuana co-marketing and the first longitudinal study of marketing in vape shops, and applies an innovative approach to spatial clustering. To enhance reach and impact, a dissemination plan will inform local, state, and federal stakeholders concerned with tobacco control and drug use prevention.